1. Technical Field
This disclosure relates to an electrophotographic photoreceptor (hereinafter referred to as a photoreceptor). In addition, the present invention also relates to a method for preparing the electrophotographic photoreceptor, and an image forming method, an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, organic photoreceptors (OPCs) have been used for various image forming apparatuses such as copiers, printers, facsimiles and multi-functional apparatuses instead of inorganic photoreceptors because of having the following advantages over inorganic photoreceptors:    (1) good optical properties such that the photoreceptors have photosensitivity over a broad wavelength range and can absorb a large amount of light;    (2) good electric properties such as high photosensitivity and stable charging property;    (3) wide material selectivity (i.e., various kinds of materials can be used for the photosensitive layer);    (4) good productivity;    (5) low costs; and    (6) little toxicity.
On the other hand, recently, image forming apparatuses are required to have a small size and to produce images at a high speed without frequently performing a maintenance operation, and therefore a need exists for a small-size photoreceptor having a good durability. In general, organic photoreceptors are soft because of having an outermost layer including a low molecular weight charge transport material and an inactive polymer. Therefore, when image forming operations such as charging, developing, transferring and cleaning operations are repeatedly performed on such organic photoreceptors, the surface of the photoreceptors is easily abraded due to the mechanical stresses applied thereto.
In addition, in order to produce high quality images, the particle size of the toners used for forming visual images in image forming apparatus becomes smaller and smaller. In order to well remove residual toner particles on the surface of the photoreceptors used for the image forming apparatuses, a cleaning blade having a high hardness is contacted with the surface of the photoreceptors at a high pressure. Thereby, abrasion of the surface of the photoreceptors is accelerated.
Abrasion of the surface of a photoreceptor deteriorates the photosensitivity and charging properties of the photoreceptor, resulting in decrease of image density and formation of abnormal images such as background development in that background of images is soiled with toner particles. If local abrasion (such as formation of scratches) is caused to the surface of a photoreceptor, the photoreceptor produces streak images due to defective cleaning.
When the surface of a photoreceptor is abraded and/or local abrasion (scratches) is caused to the surface, the photoreceptor has to be undesirably replaced with a new photoreceptor, resulting in shortening of the life of the photoreceptor. Thus, the abrasion problem is a big problem to be solved.
Therefore various attempts have been made to solve the abrasion problem of organic photoreceptors.
In attempting to improve the abrasion resistance of photoreceptors, the following techniques have been proposed.    (1) A crosslinked binder resin is used for the outermost layer of a photoreceptor (published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 56-48637);    (2) A charge transport polymer is used for the photosensitive layer of a photoreceptor (JP-A64-1728); and    (3) An inorganic filler is dispersed in the outermost layer of a photoreceptor (JP-A 04-281461).
The photoreceptor (1) using a crosslinked binder resin for the outermost layer thereof has a drawback in that the residual potential (i.e., the potential of a light-irradiated portion of the photosensitive layer) is high due to poor compatibility of the crosslinked binder resin with charge transport materials, and impurities (such as polymerization initiators, and unreacted materials and groups) included therein, and thereby low density images are produced.
The photoreceptors (2) and (3) have a relatively improved abrasion resistance. However, the abrasion resistance thereof is not so satisfactory as to satisfy the recently required durability. In addition, the photoreceptor (3) tends to have a relatively high residual potential due to carrier traps (in general, positive hole traps are formed in OPCs) present on the surface of the inorganic filler, thereby easily causing a problem in that low density images are produced.
Thus, the photoreceptors (1), (2) and (3) do not have the recently required durability (i.e., good combination of electric durability and mechanical durability).
In attempting to improve the abrasion resistance and scratch resistance of the photoreceptor (1), Japanese patent No. 3,262,488 (i.e., JP-A 08-262779) discloses a photoreceptor having a protective layer prepared using a multifunctional crosslinking acrylate monomer. Although it is described therein that a charge transport material can be included in the protective layer, there is no detailed description of the charge transport material. As a result of the present inventors' study, it is found that when a low molecular weight charge transport material is included in the protective layer, the photoreceptor causes a problem in that such a low molecular weight charge transport material has poor compatibility with a crosslinked acrylate, thereby causing problems in that the charge transport material is separated from the crosslinked acrylate while precipitated, resulting in occurrence of a blushing phenomenon, and the strength of the protective layer deteriorates.
In addition, the photoreceptor is prepared by reacting monomers while a binder resin is included in the reaction system. Therefore, problems in that the crosslinking reaction does not sufficiently proceed and the crosslinked material has poor compatibility with the binder resin occur. In this case, a phase separation phenomenon is caused in the crosslinking reaction, resulting in roughening of the surface of the resultant photoreceptor, thereby causing a defective cleaning problem.
Japanese patent No. 3,194,392 (i.e., JP-A 05-216249) proposes another technique for improving abrasion resistance of a photosensitive layer such that a charge transport layer is formed using a coating liquid including a combination of a monomer having a carbon-carbon double bond (C═C), a binder resin, and a charge transport material having a carbon-carbon double bond, which is to be reacted with the monomer. In this regard, the binder resin may have a carbon-carbon double bond to be reacted with the charge transport material. The photoreceptor has a good combination of abrasion resistance and electric properties. However, according to our study, the photoreceptor has a drawback in that when a binder resin having no reactivity is used, the binder resin has poor compatibility with the crosslinked material formed by the reaction of the monomer with the charge transport material, thereby causing a phase separation phenomenon in the crosslinking reaction, resulting in roughening of the surface of the resultant photoreceptor. Therefore, the photoreceptor causes a defective cleaning problem. In addition, the binder resin retards the crosslinking reaction of the monomers.
Further, the monomers described therein are di-functional, and therefore the crosslinking density is relatively low. Therefore, the abrasion resistance of the photoreceptor is not so satisfactory as to satisfy the recently required durability. The same is true for the case where a reactive binder resin having a carbon-carbon double bond is used. Specifically, in this case, it is impossible to increase the amount of the charge transport material while increasing the crosslinking density because the numbers of the functional groups included in the monomers and binder resins are small. Therefore, the resultant photoreceptor cannot have a good combination of electric properties and abrasion resistance.
JP-A 2000-66425 proposes a photoreceptor having a photosensitive layer including a compound prepared by crosslinking a positive hole charge transport material having two or more chain-polymerizable functional groups in a molecule. However, the crosslinked positive hole charge transport material is bulky because of having two or more chain-polymerizable functional groups and therefore the photosensitive layer tends to be strained, resulting in increase of internal stress. Therefore, the photoreceptor tends to cause problems in that the surface of the photosensitive layer is roughened and/or cracks are formed in the photosensitive layer. Namely, the photoreceptor has unsatisfactory durability.
Other crosslinked outermost layers prepared by crosslinking a radically polymerizable monomer, which has three or more functional groups and no charge transport structure, and a radically polymerizable monofunctional monomer, which has a charge transport structure, have been proposed in JP-As 2004-302450, 2004-302451, and 2004-302452. The outermost layers have good abrasion resistance, but the abrasion resistance is not so satisfactory as to satisfy the recently required excellent and long-term durability. Particularly, the adhesion between the outermost layers and the photosensitive layers deteriorates due to the internal force caused by crosslinking of the outermost layers. Therefore, when a mechanical force is applied thereto for a long period of time, a problem in that the outermost layers are peeled from the photosensitive layers tends to be caused.
Because of these reasons, a need exists for a photoreceptor which can maintain a good combination of electric properties and abrasion resistance and which can produce high quality images over a long period of time.